1. Field of the Invention
The present invention relates to a production method for a grain oriented silicon steel sheet, and specifically to a production method for a grain oriented silicon steel sheet exhibiting low core loss and high magnetic flux density.
2. Description of the Prior Art
Grain oriented silicon steel sheets are primarily utilized as core materials for transformers and various electric appliances. Such applications require core materials which exhibit excellent magnetic characteristics, i.e., high magnetic flux density and low core loss.
Conventional production methods for grain oriented silicon steel sheet involve forming a slab 100 to 300 mm thick, subjecting the slab to hot rolling after heating the slab to 1250.degree. C. or higher to form a hot-rolled sheet; cold rolling the hot-rolled sheet at least once to a final sheet thickness, with intermediate annealing(s) conducted between consecutive cold rollings; finish annealing the cold-rolled sheet for secondary recrystallization and purification, the finishing annealing being performed after subjecting the cold-rolled sheet to decarburization annealing and then applying an annealing separating agent thereon.
That is, after the slab is first heated to high temperatures to completely solubilize inhibitor components, a primary recrystallized grain structure is obtained by hot rolling, cold rolling at least once and annealing at least once, and then primary recrystallized grains are recrystallized to secondary recrystallized crystal grains of a (110) (001) direction by finishing annealing, whereby needed magnetic characteristics are secured.
In order to accelerate secondary recrystallization, it is important to control the deposition of a dispersion phase through an inhibitor. The function of the inhibitor is to inhibit the normal grain growth of primary recrystallized grains so that the dispersion phase is dispersed in the steel in a uniform manner and a suitable size, and to uniformly distribute the primary recrystallized grain structure throughout the sheet thickness at a suitable crystal grain size. Examples of inhibitors include sulfides, selenides and nitrides such as MnS, MnSe, AlN, and VN, and other materials having very small solubility in steel. Further, intergranular segregation type elements such as Sb, Sn, As, Pb, Ce, Cu, and Mo are used as inhibitors.
In order to obtain a good secondary recrystallized structure, it is important to control the deposition of the inhibitor from hot rolling to the subsequent secondary recrystallization annealing. This inhibitor deposition control is important to the realization of excellent magnetic characteristics.
Techniques described in Japanese Patent Publication No. 38-14009, Japanese Patent Application Laid-Open No. 56-33431, Japanese Patent Application Laid-Open No. 59-50118, Japanese Patent Application Laid-Open No. 64-73023, Japanese Patent Application Laid-Open No. 2-263924, Japanese Patent Application Laid-Open No. 2-274811, and Japanese Patent Application Laid-Open No. 5-295442 disclose conventional techniques which control inhibitor deposition by controlling temperature hysteresis from the finishing rolling of the hot rolling step to coiling.
Disclosed in Japanese Patent Publication No. 38-14009 is a production method for grain-oriented silicon electro-steel, comprising subjecting a hot-rolled steel strip of the grain-oriented silicon electro-steel to solution heat treatment at temperatures ranging from 790.degree. C. to 950.degree. C. to maintain carbon in the form of solid solution, quickly quenching the steel strip down to a temperature of 540.degree. C. or less in order to prevent intergranular carbides from being formed, maintaining the steel strip at temperatures of 310.degree. to 480.degree. C. during which lens-shaped deposits appear in the grains, followed by another quenching step, and then repeating cold rolling and annealing alternately in order to form a grain-oriented structure.
In this method, however, an inhibitor component is not added. Thus, this method primarily seeks to control the form of deposited carbide by controlling the cooling rate and the length of time spent in a carbide depositing temperature region (in the vicinity of 700.degree. C.). Accordingly, improved magnetic characteristics have not been realized from the actual application of this technique to the production of a grain oriented electromagnetic steel sheet containing AlN, MnSe and MnS.
Disclosed in Japanese Patent Application Laid-Open No. 56-33431 are a method involving controlling coiling temperatures in a temperature range of 700.degree. to 1000.degree. C., a method involving heating a coil for 10 minutes to 5 hours after coiling at high temperatures of 700.degree. to 1000.degree. C., and a method involving quenching the coil after coiling at high temperatures of 700.degree. to 1000.degree. C.
The technique disclosed in this publication seeks to improve the deposition-dispersion state of AlN as an inhibitor, but heterogeneous decarbonization still occurs due to self-annealing within the coil after coiling, and the subsequent formation of a cold-rolled aggregate structure is unstable, which increases scattering in the characteristics of the product. In particular, water cooling of a coil results in an uneven cooling rate and therefore becomes the primary factor behind the scattering of product characteristics.
Disclosed in Japanese Patent Application Laid-Open No. 59-50118 is a method involving the cooling of a hot-rolled steel strip to temperature ranges calculated from the following equations (a) and (b) at a cooling rate of 7.degree. to 40.degree. C./second after separation from a final finishing stand. The steel strip is then coiled and left to cool. Also disclosed is a method in which a hot-rolled steel strip is cooled to temperatures calculated from the following equation (c) or lower at a cooling rate of 7.degree. to 30.degree. C./second after separation from the final finishing stand. The steel strip is then coiled, followed by further cooling of the coiled steel strip with water. Equations (a), (b) and (c) are as follows: EQU (35.times.log V+515).degree. C. (a) EQU (445.times.log V-570).degree. C. (b) EQU (20.times.log V+555).degree. C. (c)
wherein V represents the cooling rate (.degree. C./second) of the hot-rolled steel strip during the steps of separation from the final finishing stand to coiling.
However, these methods are directed to processes where AlN is not used as an inhibitor, and such methods would be expected to negatively affect the production of a grain oriented electromagnetic steel sheet when using AlN alone or AlN and MnSe compositely.
Disclosed in Japanese Patent Application Laid-Open No. 64-73023 discloses a method involving controlling the average cooling rate from the termination of finishing rolling in the hot rolling step to coiling to 10.degree. C./second or more and less than 40.degree. C./second and controlling the range of coiling temperatures from 550.degree. to 750.degree. C. A method involving controlling the average cooling rate and the coiling temperature to 40.degree. to 80.degree. C./second and 550.degree. to 750.degree. C., respectively, is also disclosed.
As in the methods disclosed in Japanese Patent Application Laid-Open No. 59-50118, these methods utilize MnS and MnSe as inhibitors and do not relate or refer to a production method for a grain oriented electromagnetic steel sheet which utilizes AlN. Further, with respect to the disclosed cooling rates, both of these references consider only the average cooling rates in the steps of from the termination of finishing to coiling. That is, there is no consideration at all of the residence time at high temperatures immediately after the termination of rolling, which markedly affects the deposition state of AlN as an inhibitor or the composite deposition state of AlN and MnSe or MnS.
Further, disclosed in Japanese Patent Application Laid-Open No. 2-263924 is a method in which a silicon steel slab comprising 0.02 to 0.100 wt % of carbon, 2.5 to 4.5 wt % of silicon, a conventional inhibitor component, and the balance of iron and incidental impurities is subjected to hot rolling, cold rolling at a draft of 80% or more, decarburization annealing, and then final finishing annealing without subjecting the steel to hot-rolled sheet annealing to thereby manufacture a grain oriented electromagnetic steel sheet. The hot rolling terminating temperature is controlled to 750.degree. to 1150.degree. C.; the roller sheet is maintained at temperatures of 700.degree. C. or higher for at least one second or more after terminating the hot rolling; and the coiling temperature is controlled to lower than 700.degree. C.
From the viewpoint of production costs, this technique seeks to accelerate recrystallization by maintaining high temperatures after finishing rolling to thereby improve the structure, while omitting hot-rolled sheet annealing. The acceleration of recrystallization after the hot rolling with this technique improves the structure and can omit the annealing of a hot-rolled sheet, but an improved inhibitor deposition state is not obtained. Since the annealing of a hot-rolled sheet is omitted in this technique, inhibitor deposition control is sacrificed.
Further, disclosed in Japanese Patent Application Laid-Open No. 2-274811 is a method in which a slab comprising 0.021 to 0.075 wt % of carbon, 2.5 to 4.5 wt % of silicon, 0.010 to 0.060 wt % of acid soluble Al, 0.0030 to 0.000130 wt % of nitrogen, 0.014 wt % or less of selenium, 0.05 to 0.8 wt % of manganese, and the balance iron and incidental impurities is heated at temperatures of lower than 1280.degree. C. and then is subjected to hot rolling. Subsequently, the hot-rolled sheet is subjected to hot-rolled sheet annealing if necessary and then at least one cold rolling including a final cold rolling at a draft of 80% or more, with intermediate annealings being performed between consecutive cold rollings, if necessary. Then, the cold-rolled sheet is subjected to decarburization annealing and final finishing annealing to complete the production of a grain oriented electromagnetic steel sheet. During the process, the hot rolling terminating temperature is controlled to 750.degree. to 1150.degree. C.; the hot-rolled sheet is maintained at temperatures of 700.degree. C. or higher for at least one second or more after the completion of the hot rolling; and the coiling temperature is controlled to lower than 700.degree. C.
This method seeks to provide, in a production process utilizing low temperature slab heating, accelerated recrystallization by maintaining the rolled sheet at high temperatures after finishing rolling to enhance and stabilize the magnetic characteristics. However, while the solution of AlN is possible with the low temperature slab heating, the solution of MnS and MnSe can not sufficiently be achieved. In particular, in the case where such hot rolling and cold rolling as described above are applied to a production method in which high temperature slab heating is carried out to sufficiently solubilize inhibitors, products having excellent magnetic characteristics can not be produced because of a difference in the deposition states of the inhibitors. That is, since inhibitor control does not occur during low temperature slab heating, products having excellent magnetic characteristics cannot be stably produced.
Further, disclosed in Japanese Patent Application Laid-Open No. 5-295442 is a method in which a steel sheet after hot rolling is subjected to cold rolling at a final cold rolling draft of 80% or more, wherein the relation between the Ti content and the average cooling rate Ta (.degree. C./second) at temperatures of 850.degree. C. or lower and up to 600.degree. C. after emerging from a finishing stand for hot rolling is:
when Ta.gtoreq.30.degree. C./second and Ti.ltoreq.0.003 weight %, PA1 Ta.gtoreq.-7/3Ti+100, PA1 when 0.003&lt;Ti.ltoreq.0.008 weight %, PA1 Ta.ltoreq.-11/5T+206, PA1 Ta: .degree. C./sec PA1 Ti: 10.sup.-4 weight %.
However, Ti remaining in a product produced by this method forms oxides and nitrides, resulting in core loss age degradation.
Conventional techniques have not considered the heat hysteresis of a steel sheet from the termination of hot finishing rolling up to coiling in order to disperse an inhibitor in steel in an even form and in a suitable size.
Methods for controlling the cooling rate from the termination of hot finishing rolling up to coiling (for example, as disclosed in Japanese Patent Application Laid-Open No. 59-50118) are known. However, this method has not been directed to the control of an inhibitor, but rather to the deposition of fine carbides. Further, known methods for controlling the cooling rate from the termination of hot finishing rolling up to coiling controls only the average cooling rate. In particular, there has been no consideration given to cooling immediately after the completion of hot finishing rolling.
The conventional techniques described above have not achieved the effective deposition control of an inhibitor. This has made it impossible to manufacture through conventional techniques a grain oriented silicon steel sheet which exhibits excellent magnetic flux density and core loss value.